While melanoma immunotherapy is starting to show beneficial results in the clinic, only a subset of patients currently benefit from this form of treatment. The development of these cancer immunotherapy approaches has primarily focused on enhancing the function of the T cell while much less work has been devoted to developing strategies to enhance antigen-presenting cell activity. Dendritic cells (DCs) are potent antigen- presenting cells that are capable of both activating tumor antigen-specific T cells and directing their function. It is believed that the limited efficacy of current immunotherapy agents is largely due to the evolution of methods utilized by developing cancers to avoid detection and destruction by the host immune system. Recent work has shown that cancers are capable of actively tolerizing local DCs, enabling them to promote the expansion of regulatory T cells (Tregs) and maintain a state of immune tolerance. One important component of tumor immune evasion has been determined to be the indoleamine 2,3-dioxygenase (IDO) immunoregulatory enzyme that converts the tryptophan amino acid into kynurenine. This process suppresses conventional T cell activity and drives the development of Tregs. The molecular mechanisms utilized by melanomas and other solid tumors to promote this DC tolerization process remain unknown. Previous work has shown that melanomas express high levels of several Wnt ligands and that Wnt5a expression has been associated with a poor prognosis. We have determined that Wnt5a is the dominant factor driving DC-dependent differentiation and expansion of Tregs within the melanoma microenvironment. Although this process is partially dependent on Wnt5a-mediated upregulation of DC IDO expression, our preliminary data now suggests that this DC tolerization program is comprised of additional alterations in DC metabolism. We are now proposing a strategy to extend these studies by 1) characterizing the effect that this novel Wnt5a--catenin-dependent pathway has on anti-melanoma immunity and melanoma progression, 2) determining if the Wnt5a--catenin signaling pathway contributes to immunotherapy resistance and whether this process can be pharmacologically reversed, and 3) elucidating the metabolic alterations induced by Wnt5a in DCs and how these metabolic changes contribute to generating an immunotolerant microenvironment. To achieve these aims, we plan to utilize a genetically engineered mouse model that physiologically resembles human melanoma and allows for a more in-depth analysis of the interplay between a developing malignancy and the local immune response. This project seeks to define a novel immune evasion pathway and, in doing so, offers a strategy to both enhance the efficacy of currently available immunotherapy approaches and broaden the patient population that may benefit from tumor immunotherapy.